KENNEDY SPACE CENTER, FLA. - Space Shuttle Discovery, atop the Mobile Launcher Platform, slowly rolls out of the Vehicle Assembly Building (VAB) at NASA?s Kennedy Space Center. First motion was at 2:04 p.m. EDT. The Shuttle comprises the orbiter, External Tank (ET) and twin Solid Rocket Boosters (SRBs). The Mobile Launcher Platform moved by the Crawler-Transporter underneath. The Crawler is 20 feet high, 131 feet long and 114 feet wide. It moves on eight tracks, each containing 57 shoes, or cleats, weighing one ton each. Loaded with the Space Shuttle, the Crawler can move at a maximum speed of approximately 1 mile an hour. A leveling system in the Crawler keeps the Shuttle vertical while negotiating the 5 percent grade leading to the top of the launch pad. Launch of Discovery on its Return to Flight mission, STS-114, is targeted for May 15 with a launch window that extends to June 3. During its 12-day mission, Discovery?s seven-person crew will test new hardware and techniques to improve Shuttle safety, as well as deliver supplies to the International Space Station. Discovery was moved on March 29 from the Orbiter Processing Facility to the VAB and attached to its propulsion elements, a redesigned ET and twin SRBs.